Method for braking a vehicle by means of a fluidically triggered vehicle brake system, and vehicle brake system

ABSTRACT

In a method for braking a vehicle by means of a fluidically triggerable vehicle brake system, wherein the vehicle brake system comprises a respective fluidically triggerable brake unit that is assigned to a vehicle wheel and is fluidically coupled to a brake force generator via at least one fluid circuit, wherein a pumping mechanism by means of which at least one of the brake units can be fed with brake fluid regardless of whether the brake force generator is activated, is provided in the at least one fluid circuit in order to convey brake fluid, and wherein control valves by means of which the brake force generator can be fluidically coupled to and disconnected from the brake units and the pumping mechanism are provided in the fluid circuit, it is provided that in order to comfortably create a parking brake condition, the following steps are carried out:
     A) a fluid pressure is built up in the at least one fluid circuit via the brake force generator such that at least two of the brake units are fluidically triggered;   B) the brake force generator is disconnected from the brake units and the pumping mechanism by closing the control valves;   C) the pumping mechanism is activated (in two) and brake fluid is conveyed from one of the at least two brake units to the other one of the at least two brake units, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage of International Application No.PCT/EP2005/005455 filed May 19, 2005, the disclosures of which areincorporated herein by reference, and which claimed priority to GermanPatent Application No. 10 2004 025 402.8 filed May 24, 2004, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method for braking a vehicle by meansof a fluidically triggerable vehicle brake system and to a correspondingvehicle brake system, wherein the vehicle brake system comprises arespective fluidically triggerable brake unit that is assigned to avehicle wheel and is fluidically coupled to a brake force generator viaat least one fluid circuit, wherein a pumping mechanism by means ofwhich at least one of the brake units can be fed with brake fluidregardless of whether the brake force generator is activated, isprovided in the at least one fluid circuit in order to convey brakefluid, and wherein control valves by means of which the brake forcegenerator can be fluidically coupled to and disconnected from the brakeunits and the pumping mechanism are provided in the fluid circuit.

A vehicle brake system of this type is known from the prior art. Thusdocument DE 101 10 658 C1 and corresponding U.S. Pat. No. 6,672,686,which are hereby incorporated by referenced, disclose a hydraulicvehicle brake system in which two separate fluid circuits can be fedwith brake fluid via a master brake cylinder. Two brake units areassigned to each fluid circuit, one brake unit of a front wheel and onebrake unit of a rear wheel. To trigger the respective brake unitscontrol valves are provided. A pumping mechanism is also assigned to thetwo fluid circuits, via which the brake units can be triggered via themaster brake cylinder independently of a pressure generation. It isconsequently possible to achieve electronic control systems, such as anelectronic stability program (ESP).

In order to be able to a achieve parking brake effect withoutsignificant additional technical complexity, in addition to thefunctionalities, described for the case of service braking in documentDE 101 10 658 C1 and U.S. Pat. No. 6,672,686, of a vehicle brake systemof this type, it is also known from the prior art to use the fluidpressure built up via the master brake cylinder and acting on theindividual brake units to create a parking brake condition. For thispurpose the brake fluid fed to the brake units of the vehicle wheels ofthe front axle for example is conveyed via the pumping mechanism fromthe brake units of the front axle to the brake units of the rear axleand then mechanically locks the brake units of the rear axle. Inconventional vehicle brake systems this takes place in a condition inwhich a specific brake pressure is still built up via the master brakecylinder, for example in that the brake pedal is firmly depressed by thedriver of the vehicle. However, this in turn means that duringactivation of the parking brake effect there is a direct fluidicconnection between the master brake cylinder and the brake units. As aresult vibrations and noise which occur during activation of the pumpingmechanism to cause the parking brake condition are transmitted directlyvia the master brake cylinder, the adjoining brake booster and the brakepedal into the interior of the vehicle, and the driver and additionalvehicle occupants can regard this as being disruptive. As a result ofactivation of the pumping mechanism brake fluid is also inevitablyremoved from the master brake cylinder, so the brake pedal yields underthe pedal actuating force exerted by the driver of the vehicle, and thisis again felt by the driver and could be regarded as unpleasant or atleast unfamiliar.

BRIEF SUMMARY OF THE INVENTION

Compared with this, an aspect of this invention is to preferably providea method and a vehicle brake system of the type mentioned at the outsetin which activation of the parking brake function can be achieved usingsimple technical means and by avoiding disruptive influences in theinterior of the vehicle.

This may be is achieved by a method of the type mentioned at the outset,wherein to create a parking brake condition, the following steps arecarried out:

A) a fluid pressure is built up in the at least one fluid circuit viathe brake force generator such that at least two of the brake units arefluidically triggered;

B) the brake force generator is disconnected from the brake units andthe pumping mechanism by closing the control valves;

C) the pumping mechanism is activated and brake fluid is conveyed fromone of the at least two brake units to the other one of the at least twobrake units, respectively.

According to one aspect of the invention as already described at theoutset in relation to the prior art, firstly a fluid pressure is builtup in the at least one fluid circuit via the brake force generator,which pressure can be detected by means of a fluid sensor, so a brakingeffect is achieved at the at least two brake units, i.e. the brake unitsare applied. Consequently however the fluidic connection between thebrake force generator and the remainder of the respective fluid circuitis broken, so activation of the pumping mechanism and resultantvariations in pressure and vibrations in the fluid circuit generallycannot spread into the brake force generator. Only after closing thecontrol valves is the pumping mechanism generally activated and as aconsequence brake fluid is conveyed via the pumping mechanism from onebrake unit of the fluid circuit to the respective other brake unit ofthe fluid circuit. The method according to the invention therefore mayensure that the effect of the pumping mechanism does not lead todisruptive noises in the interior of the vehicle or to unfamiliaryielding of the brake pedal.

Where within the framework of the description and claims of the presentinvention reference is made to a “brake force generator”, thisexpression is to include both an arrangement in which a fluid pressureis generated in the at least one fluid circuit by a brake pedal, a brakebooster and an adjoining master brake cylinder, as well as anarrangement in which a brake pedal actuation is detected and accordingto the requirement thereof a fluid pressure is generated in the at leastone fluid circuit without directly using the pedal actuating forceexerted on the brake pedal. The expression “brake force generator”should also include arrangements which only partially use the pedalactuating force exerted on the brake pedal, or use it only in certain(emergency) operating situations.

A development of the present invention provides that the pumpingmechanism is triggered according to specific parameters, for example thevehicle inclination or/and the vehicle weight or/and the rotationalspeed of the wheels or/and the operating setting of a parking brakeswitch or/and the brake pedal actuation or/and the yaw rate or/and thetransverse acceleration or/and the fluid pressure generated by the brakeforce generator. With respect to activation of the parking brakefunction it is thus possible for this to be prevented until therotational speed output by each of the wheel sensors is at a specificlimiting value or has even sunk to zero. It is also possible, using thevehicle inclination and the current vehicle speed, to determine aminimum fluid pressure at which accidental rolling away of the vehicleis ruled out.

As already indicated above, it may be necessary as a function of thecurrent operating situation of the vehicle, for example when parking thevehicle on a very steep roadway, for the fluid pressure at thefluidically triggered brake units to achieve the parking brake effect tobe considerably increased. In order to be able to provide sufficienthydraulic fluid, which is then conveyed via the pumping mechanism to therespectively triggered brake unit, a development of the inventionprovides that at least one fluid accumulator is assigned to each of thefluid circuits. The fluid accumulator can thus in each case be filledwith brake fluid in conditions in which there is a sufficiently highfluid pressure, wherein this brake fluid can then be removed again asrequired. It is thus possible according to the invention for (optionallypressurised) brake fluid to be temporarily stored in the fluidaccumulator and to be conveyed in step C) via the pumping mechanism tothe respective other brake unit.

If the operating situation already discussed above is taken up again inwhich the vehicle is to be parked on a steep road, the situation mayoccur where the driver notices that the pedal actuating force he exertson the brake pedal and the resultant fluid pressure in the at least onefluid circuit is not sufficient to safely park the vehicle. For example,the vehicle continues to roll at low speed despite the brake pedal beingdepressed. In this condition the driver will continue to depress thebrake pedal in order to build up a higher fluid pressure in the at leastone fluid circuit and ultimately increase the braking force. To takeaccount of this operating situation, a development of the inventionprovides that when the fluid pressure is increased by the brake forcegenerator during step B), the control valves are triggered in such a waythat additional brake fluid from the brake force generator is stored inthe fluid accumulator and the brake force generator is then disconnectedfrom the brake units and the pumping mechanism by closing the controlvalves. As a result of this measure according to the invention the fluidpressure additionally generated in the brake force generator cantherefore also be used to activate the parking brake function.

In this connection an advantageous development of the invention providesthat the control valves are triggered according to a pulse method, forexample a pulse-width modulation method. The opening or closing pulsewidth of the respective control valve may be varied within a period oftime owing to a pulse-width modulation method of this type. Consequentlyit is possible to close the respective control vale with a holding forcewhich corresponds to the fluid pressure at the start of activation ofthe parking brake function.

With respect to the pumping mechanism, it may be provided according tothe invention that it is constructed as a recirculating pump in thefluid circuit, in particular as part of an electronic control system,for example an electronic stability system or/and an anti-lock brakingsystem or/and a traction control system or/and an automatic speedcontrol system. A pumping mechanism that exists in the braking systemanyway may therefore be used to achieve the invention.

An advantageous embodiment of the invention provides that two fluidcircuits are provided, wherein each fluid circuit is fluidically coupledto a brake unit assigned to a front wheel and to a brake unit assignedto a rear wheel respectively. It may also be provided that the vehiclewheels respectively assigned to a fluid circuit are arranged diagonallyon the vehicle. In this connection it may also be provided according tothe invention that in step C) brake fluid is conveyed from the frontwheel to the rear wheel in each case.

The invention also relates to a vehicle brake system comprising arespective fluidically triggerable brake unit that is assigned to avehicle wheel and is fluidically coupled to a brake force generator viaat least one fluid circuit, wherein a pumping mechanism by means ofwhich at least one of the brake units can be fed with brake fluidregardless of whether the brake force generator is activated, isprovided in the at least one fluid circuit in order to convey brakefluid, and wherein control valves by means of which the brake forcegenerator can be fluidically coupled to and disconnected from the brakeunits and the pumping mechanism are provided in the fluid circuit. Inthis vehicle brake system it is provided according to the invention thatin order to create a parking brake condition, the brake force generatorbuilds up a fluid pressure in the at least one fluid circuit in order tofluidically trigger at least two of the brake units, in that the controlvalves can also be controlled in such a way that the brake forcegenerator is fluidically disconnected from the brake units and thepumping mechanism, and in that following fluidic decoupling of the brakeforce generator, the pumping mechanism conveys brake fluid from one ofthe at least two brake units to the other one of the at least two brakeunits respectively.

According to a development of the present invention the vehicle brakesystem comprises sensors for determining the vehicle inclination or/andthe vehicle weight or/and the rotational speed of the wheels or/and anoperating setting of a parking brake switch or/and the current excursionof a brake pedal or/and the transverse acceleration of the vehicleor/and the yaw rate or/and the fluid pressure generated by the brakeforce generator.

The vehicle brake system according to the invention is constructed insuch a way that at least one fluid accumulator is assigned to each ofthe fluid circuits.

Other advantages of this invention will become apparent to those skilledin the art from the following detailed description of the preferredembodiments, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a vehicle brake system according to theinvention and

FIG. 2 shows a flow diagram to describe the method according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of a vehicle brake system according to theinvention generally designated by reference numeral 10. The systemcomprises a first fluid circuit 12 and a second fluid circuit 14. Thetwo fluid circuits 12 and 14 are fluidically coupled to a tandem masterbrake cylinder 16, wherein the fluid circuit 12 is fed with brake fluidby displacement of a secondary piston 18 from a secondary pressurechamber 20 and wherein the fluid circuit 14 is fed with brake fluid bydisplacement of a primary piston 22 from a primary pressure chamber 24.The main brake cylinder 16 is conventionally coupled to a vacuum brakebooster 26 which can be actuated by a brake pedal (not shown).

The fluid circuits 12 and 14 are fluidically connected to brake units28, 30, 32, 34, wherein the brake unit 28 is assigned to the left rearwheel, brake unit 30 to the right front wheel, brake unit 32 to the leftfront wheel and brake unit 34 to the right rear wheel of the vehicle.This assignment is what is referred to as a diagonal distribution. Otherdistributions could equally exist according to the invention, forexample a front axle-rear axle distribution.

A respective intake valve 36, 38, 40, 42 is assigned to each brake unit28, 30, 32, 34 and can be triggered separately via an electronic controlunit (not shown). A respective exhaust valve 44, 46, 48, 50 is alsoassigned to each brake unit 28, 30, 32, 34 and can also be triggeredseparately via the electronic control unit. The two fluid circuits 12and 14 are fluidically coupled to a pumping mechanism 52. An ABS or ESPfunctionality for example may be implemented via the intake valves 36,38, 40, 42, the exhaust valves 44, 46, 48, 50 and the pumping mechanism52. The intake valves 36, 38, 40, 42 are triggered with the aid of a PWM(pulse-width modulation) method. This results in high variability intriggering of these control valves.

Each of the fluid circuits 12 and 14 is also provided with a respectivefluid accumulator 54 and 56 in which brake fluid can optionally betemporarily stored under pressure.

Finally a pair of control valves 58 and 60 and 62 and 64 is assigned toeach of the fluid circuits 12 and 14. The control valves 58, 60, 62 and64 may also be triggered separately via the electronic control unitusing a PWM (pulse-width modulation) method, so high variability intriggering of these control valves may be achieved. The control valves58, 60, 62 and 64 are arranged as follows in the two fluid circuits 12and 14: control valves 58 and 60 are assigned to fluid circuit 12 andthus to brake units 28 and 30; control valves 62 and 64 are assigned tofluid circuit 14 and thus to brake units 32 and 34. This arrangementmakes it possible to fluidically disconnect the individual brake units28, 30, 32 and 34 from the master brake cylinder 16, so activation ofthe pumping mechanism 52 and resultant recirculation of the brake fluidinto the individual fluid circuits 12 and 14 has no repercussion for themaster brake cylinder 16.

FIG. 1 also shows that a pressure sensor 66 is arranged in the fluidcircuit 14. FIG. 1 also shows a series of check valves and twopressure-limiting valves 68 and 70 of which one is assigned to fluidcircuit 12 and the other to fluid circuit 14. These pressure-limitingvalves 68 and 70 are used as overload protection.

In an alternative embodiment, instead of by way of the additionalpressure-limiting valves 68 and 70, the overload function can beintegrally achieved by mechanical or/and electronic measures at controlvalves that exist anyway.

The mode of operation of the vehicle brake system 10 during servicebraking, i.e. during deceleration of the vehicle during travel, isdescribed in detail in DE 101 10 658 C1 already mentioned at the outset,and will therefore not be described in detail again here. The content ofDE 101 10 658 C1 is also to be incorporated into this disclosure byreference thereto.

The mode of operation of the vehicle brake system according to theinvention and the course of the method according to the invention onactivation of the parking brake function will be described hereinafterwith reference to FIG. 2.

Firstly the system is started in step S1, for example by the driveractuating the brake pedal according to step S2 and pressing a parkingbrake button in the interior of the vehicle according to step S3. Onactuation of the brake pedal a fluid pressure is built up in the fluidcircuits 12 and 14 via the brake force generator that comprises themaster brake cylinder 16 and brake booster 26. This fluid pressure isdetected by means of pressure sensors 66 in the fluid circuit 14 and bymeans of a corresponding pressure sensor (not shown) in fluid circuit12, as indicated in step S4, and stored in an electronic control unit(not shown). A check is then made in step S5 as to whether all startconditions are satisfied, for example whether the brake pedal has beenactuated by the driver, whether the car is still stationary or/andwhether the gear lever of a vehicle equipped with an automatictransmission is in the “Park” position.

If the start conditions are satisfied control parameters, such as thecurrent inclination of the vehicle, the total weight of the vehicle, therotational speed at the individual wheels, the current excursion of thebrake pedal, the transverse acceleration of the vehicle, the yaw rate orthe like, are consequently loaded in step S6 from the electronic controlunit (not shown in FIG. 1. These control parameters are detected byadditional sensors (not shown in FIG. 1). On the basis of thisinformation parameters for controlling the control valves 60 and 62 arecalculated, for example the pulse width of a pulse-width modulationmethod with which the control valves 60 and 62 are triggered. In step S7the control valves 60 and 62 are closed using the calculated pulse widthaccording to the pulse-width modulation method. The control valves 58and 64 are also closed.

The pumping mechanism 52 is then activated according to step S8. Withactivation of the pumping mechanism 52 the intake valves 38 and 40 ofthe brake units 30 and 32 assigned to the front wheels are closed instep S9 with the aid of a pulse-width modulation method with a holdingforce which corresponds to the fluid pressure at the start of activationof the parking brake function.

In step S10 monitoring is carried out as to whether the driver increasesthe fluid pressure in the fluid circuits 12 and 14 further by strongeror renewed depression of the brake pedal. Such an increase in pressurewould for example take place if the driver wished to park the vehicle onan inclined road, depresses the brake pedal to a certain extent for thispurpose and notices that despite the already depressed brake pedal thevehicle still does not quite stop. In such a situation the driver willdepress the brake pedal more forcefully, so the vehicle definitely stopson the inclined road.

If it is detected in step S10 that the fluid pressure is being increasedfurther by the driver, an additional quantity of brake fluid is conveyedfrom the master brake cylinder 16 into the brake units 28, 30, 32, and34 and then into the fluid accumulators 54 and 56, wherein the pulsewidth of the intake valves 38 and 40 of the brake units 30 and 32assigned to the front wheels is matched to the current fluid pressure.

If no further increase in pressure is detected in step S10, the systemproceeds to the next step, S11. In this step the exhaust valves 46 and48 of the brake units 30 and 32 assigned to the front wheels are openedand brake fluid discharged into the fluid accumulators 54 and 64.

In step S12 the electronic control unit calculates, by taking intoaccount the remaining control parameters loaded in step S6, whether thevolume of brake fluid stored in the fluid accumulators 54 and 56 issufficient to build up a sufficiently large fluid pressure in the brakeunits 28 and 34 so a reliable parking brake effect is achieved.

The intake valves 36 and 42 of the brake units 28 and 34 assigned to therear wheels are consequently opened and the intake valves 36 and 42 ofthe brake units 30 and 32 assigned to the front wheels closed. Theexhaust valves 44 and 50 of the brake units 28 and 34 assigned to therear wheels are also closed and the exhaust valves 46 and 48 of thebrake units 30 and 32 assigned to the front wheels are opened, todischarge brake fluid into the fluid accumulators 54 and 56. With thisposition of the intake valves 36, 38, 40, 42 and of the exhaust valves44, 46, 48, 50, and with a simultaneously closed position of the controlvalves 58, 60, 62, 64, in step S13 the pumping mechanism 52 conveysbrake fluid from the brake units 30 and 32 assigned to the front wheelsto the brake units 28 and 34 assigned to the rear wheels, so a fluidpressure is built up in the brake units 28 and 34 which leads toadequate application of the brake units 28 and 34.

In step S14 the system checks whether the brake fluid diverted from thebrake units 30 and 32 of the front axle into the brake units 28 and 34of the rear axle is sufficient to achieve a reliable parking brakeeffect. If it transpires that this is not the case, the system jumps tostep S8 again and additional brake fluid is conveyed from the masterbrake cylinder 16 into the fluid accumulators 54 and 56. If, bycontrast, step S14 shows that the volume of brake fluid stored in thefluid accumulators 54 and 56 is sufficient to activate a reliableparking brake function, the brake units 28 and 34 are blocked in stepS15 in the conventional manner, for example by respective blocking of amechanical parking brake arrangement. The parking brake function isaccordingly activated, so the vehicle is safely parked in step S16.

To deactivate the parking brake function the brake pedal merely has tobe strongly depressed again or an algorithm analogous to theabove-described method for activating the parking brake function can bestarted. In both cases the mechanical parking brake arrangement of thebrake units 28 and 34 is cancelled. As a result the vehicle brake systemis again in a condition in which normal service braking operations canbe carried out.

The invention described above makes it possible to use a fluidicallyactuable vehicle brake system to activate a parking brake function,wherein activation of the pumping mechanism required for this does notlead to disruptive noises in the interior of the vehicle and cannot besensed by the driver by way of unfamiliar yielding of the brake pedaleither.

In accordance with the provisions of the patent statutes, the principleand mode of operation of this invention have been explained andillustrated in its preferred embodiment. However, it must be understoodthat this invention may be practiced otherwise than as specificallyexplained and illustrated without departing from its spirit or scope.

1. Method for braking a vehicle by means of a fluidically triggerablevehicle brake system, wherein the vehicle brake system comprises first,second, third, and fourth brake units assigned to a vehicle wheel andare fluidically coupled to a brake force generator via at least onefluid circuit, wherein a pumping mechanism by means of which at leastone of the first, second, third, and fourth brake units can be fed withbrake fluid regardless of whether the brake force generator isactivated, is provided in the at least one fluid circuit in order toconvey brake fluid, and wherein control valves by means of which thebrake force generator can be fluidically coupled to and disconnectedfrom the first, second, third, and fourth brake units and the pumpingmechanism are provided in the fluid circuit, wherein in order to createa parking brake condition, the following steps are carried out: A) afluid pressure is built up in the at least one fluid circuit via thebrake force generator such that at least the first and second brakeunits are fluidically triggered; B) the brake force generator isdisconnected from the first, second, third, and fourth brake units andthe pumping mechanism by closing the control valves; C) the pumpingmechanism is activated and brake fluid is conveyed from the first brakeunit to the third brake unit, and wherein brake fluid is conveyed fromthe second brake unit to the fourth brake unit.
 2. Method according toclaim 1, wherein at least one fluid accumulator is assigned to each ofthe at least one fluid circuits, respectively.
 3. Method according toclaim 2, wherein brake fluid is temporarily stored in the at least onefluid accumulator and in step C) is conveyed by the pumping mechanism tothe third and fourth brake units.
 4. Method according to claim 2,wherein when the fluid pressure is increased by the brake forcegenerator during step B), the control valves are triggered in such a waythat additional brake fluid from the brake force generator is stored inthe fluid accumulator and the brake force generator is then disconnectedfrom the first, second, third, and fourth brake units and the pumpingmechanism by closing the control valves.
 5. Method according to claim 1,wherein the pumping mechanism is triggered according to specificparameters including at least one of the vehicle inclination, thevehicle weight, the rotational speed of the wheels, the operatingsetting of a parking brake switch, the brake pedal actuation, the yawrate, the transverse acceleration, and the fluid pressure generated bythe brake force generator.
 6. Method according to claim 1, wherein thecontrol valves are triggered according to a pulse-width modulationmethod.
 7. Method according to claim 1, wherein the pumping mechanism isconstructed as a recirculating pump in the fluid circuit.
 8. Methodaccording to claim 7, wherein the recirculating pump is part of anelectronic control system defined by at least one of an electronicstability system, an anti-lock braking system, a traction controlsystem, and an automatic speed control system.
 9. Method according toclaim 1, wherein vehicle wheels respectively assigned to a fluid circuitare arranged diagonally on the vehicle.
 10. Method according to claim 1,wherein in step C) brake fluid is conveyed from a front wheel to a rearwheel in each case.
 11. Method according to claim 1, wherein first andsecond fluid circuits are provided, wherein the first fluid circuit isfluidically coupled to the first and third brake units, and wherein thesecond fluid circuit is fluidically coupled to the second and fourthbrake units, and wherein the first and second brake units are assignedto front wheels, and wherein the third and fourth brake units areassigned to rear wheels.
 12. Vehicle brake system comprising fluidicallytriggerable first, second, third, and fourth brake units each assignedto a respective vehicle wheel and is fluidically coupled to a brakeforce generator via at least one fluid circuit, wherein a pumpingmechanism by means of which at least one of the first, second, third,and fourth brake units can be fed with brake fluid regardless of whetherthe brake force generator is activated, is provided in the at least onefluid circuit in order to convey brake fluid, and wherein control valvesby means of which the brake force generator can be fluidically coupledto and disconnected from the first, second, third, and fourth brakeunits and the pumping mechanism are provided in the fluid circuit,wherein in order to create a parking brake condition, the brake forcegenerator builds up a fluid pressure in the at least one fluid circuitin order to fluidically trigger at least the first and second brakeunits, in that the control valves are adapted to be triggered in such away that the brake force generator is fluidically disconnected from thefirst, second, third, and fourth brake units and the pumping mechanism,and in that following fluidic decoupling of the brake force generator,the pumping mechanism conveys brake fluid from the first brake unit tothe third brake unit, and wherein brake fluid is conveyed from thesecond brake unit to the fourth brake unit.
 13. Vehicle brake systemaccording to claim 12, including a sensor for determining one of thevehicle inclination, the vehicle weight, the rotational speed of thewheels, an operating setting of a parking brake switch, the currentexcursion of a brake pedal, the transverse acceleration, the yaw rate ofthe vehicle, and the fluid pressure generated by the brake forcegenerator.
 14. Vehicle brake system according to claim 12, wherein atleast one fluid accumulator is assigned to each of the at least onefluid circuits, respectively.
 15. Vehicle brake system according toclaim 12, wherein the control valves can be triggered according to apulse-width modulation method.
 16. Vehicle brake system according toclaim 12, wherein first and second fluid circuits are provided, whereinthe first fluid circuit is fluidically coupled to the first and thirdbrake units, and wherein the second fluid circuit is fluidically coupledto the second and fourth brake units, and wherein the first and secondbrake units are assigned to front wheels, and wherein the third andfourth brake units are assigned to rear wheels.
 17. Vehicle brake systemaccording to claim 12, wherein the vehicle wheels respectively assignedto a fluid circuit are arranged diagonally on the vehicle.
 18. Vehiclebrake system according to claim 12, wherein each of the first, second,third, and fourth brake units can be fed with brake fluid via respectiveintake valves, wherein the intake valves can be triggered according to apulse-width modulation method.
 19. Vehicle brake system according toclaim 12, wherein the pumping mechanism is constructed as arecirculating pump in the fluid circuit.
 20. Vehicle brake systemaccording to claim 19 including an electronic control system defined byat least one of an electronic stability system, an anti-lock brakingsystem, a traction control system, and an automatic speed controlsystem, and wherein the recirculating pump is part of the electroniccontrol system.